205. _Symbiosis._--Very many Radiolaria, but by no means all members of this class, live in a definite commensal relation with yellow unicellular Algæ of the group Xanthellæ. In the ACANTHARIA they live within the central capsule (_Zooxanthella intracapsularis_, § 76), in the SPUMELLARIA and NASSELLARIA, on the other hand, within the calymma but outside the central capsule (_Zooxanthella extracapsularis_, § 90); in the PHÆODARIA a special form of these symbiotic unicellular Algæ appears to inhabit the phæodium in the extracapsulum, and to compose a considerable portion of the phæodellæ (_Zooxanthella phæodaris_, § 90, or better perhaps _Zoochlorella phæodaris_, § 89). Undoubtedly this commensal life is in very many cases of the greatest physiological significance for both the symbiontes, for the animal Radiolarian cells furnish the inquiline Xanthellæ not only with shelter and protection, but also with carbon dioxide and other products of decomposition for their nutriment; whilst on the other hand the vegetable cells of the Xanthellæ yield the Radiolarian host its most important supply of nutriment, protoplasm and starch, as well as oxygen for respiration. Hence it is not only theoretically possible, but has been experimentally proved, that Radiolaria which contain numerous Xanthellæ can exist without extraneous nutriment for a long period in closed vessels of filtered sea-water, kept exposed to the sunlight; the two symbiontes furnish each other mutually with nourishment, and are physiologically supplementary to each other by reason of the opposite nature of their metastasis. This symbiosis is not necessary, however, for the existence of the Radiolaria; for in many species the number of Xanthellæ is very variable and in many others they are entirely wanting.